Diode-lasers are commonly used as sources of illumination in various graphics applications such as display systems, optical printing systems and optical recording systems. By way of example, in one type of prior-art imaging system, a linear (one-dimensional) array of light modulators is illuminated by an illuminator including a light source. Illumination from the array of modulators is projected onto a recording medium or the like in the form of a line of images of the modulators. By scanning the recording medium past the line of images and appropriately synchronizing the scanning with operation of the modulator array, the modulator array is used to draw a two-dimensional image on the recording medium.
A preferred light-source for the illuminator is a linear array of diode-lasers commonly referred to as a diode-laser bar. A diode-laser bar can efficiently provide relatively high illumination power, for example about 60 Watts (W) or more, from a source having a maximum overall dimension no greater than about 10 millimeters. A disadvantage, however, is that each of the diode-lasers in the bar is an individual emitter. This presents problems in selecting an appropriate optical configuration for the illuminator.
This problem is addressed in one prior-art illuminator by including a diode-laser array (or correspondingly an array of light-emitting diodes) having a number of emitters equal to the amount of modulators to be illuminated, and an optical system configured to image each emitter onto a corresponding modulator. This type of illuminator has a disadvantage in that it is dependent on all emitters continuing to function. Failure of one emitter could cause at least a reduction in performance of the recording system, for example, a black line on an image in the direction of scanning.
An illuminator apparatus for overcoming this disadvantage is disclosed in U.S. Pat. No. 6,433,934 (Reznichenko et al.). Here, an optical system is used to project elongated, overlapping images of each individual emitter onto a modulator array. Each emitter in a high power diode-laser bar has an emitting aperture having a height of about 1 micrometer (μm) in a so called fast-axis of the bar (perpendicular to the length of the bar) and a width between about 50 and 150 in a so called slow-axis parallel to the length of the bar. The images have a length (the length of the projected line) in the slow axis and a height in the fast axis. In this kind of system, the number of emitters need not correspond to the number of modulators, and failure of an individual emitter is described as merely reducing the overall illumination on the modulator rather than effecting the spatial distribution of illumination on the modulator.
FIG. 1 is a slow-axis view of the system 20 of Reznichenko et al. This drawing is reproduced in part from the '934 patent, with some added notation, and with some different reference numerals. A diode-laser bar 22 has a plurality (here seventeen) of emitters 24. Each emitter 24 emits a beam 26 having a slow-axis divergence half-angle ω. Beams 26 are collimated in the fast-axis (not shown) by a cylindrical lens 28 that has no effect on the slow-axis divergence of the beams. Two spaced apart arrays 30 and 32 of cylindrical microlenses 31 and 32 respectively are located in the path of beams 26. Each cylindrical microlens has optical power (here positive) in the slow-axis only. In FIG. 1, and in other drawings discussed hereinbelow, the slow-axis, the fast-axis, and the propagation direction (propagation-axis) are designated as respectively the X-axis, Y-axis, and Z-axis of a Cartesian axis system.
In each microlens array there are as many microlenses as there emitters in the diode-laser bar. Each microlens in each array is aligned with a corresponding emitter. The axial spacing of the lenses and the physical aperture of the lenses in the slow axis is selected such that the entire width W of a beam 26 entering a microlens 31 of array 30 can enter the microlens. Microlenses 31 collimate the beams 26 in the slow axis, and corresponding microlenses 33 of array 32 form intermediate near-field images of the emitting apertures in a plane 35. Three of such images are designated A, B, and C in FIG. 1. Because of this arrangement, the chief rays 36C of converging bundles of rays forming the images and, correspondingly, of diverging bundles 36 of rays leaving the images are parallel to each other.
Bundles 36 pass through a cylindrical lens 34 having optical power in the fast axis only and are received by a spherical lens 38 having equal (positive) optical power in both fast and slow-axes. A front focal plane of lens 38 is arranged to be coplanar with plane 35 in which the intermediate images A, B, and C are formed. This, combined with the chief rays of bundles 36 being parallel to each other, places the images in a telecentric arrangement with lens 38. A result of this is that each slow-axis diverging ray-bundle 36 is converted into a corresponding parallel (collimated) ray-bundle 37. Ray-bundles 37 traverse a cylindrical lens 40, having optical power in the fast-axis only, and intersect in a rear (back) focal plane 42 of lens 38 to form (together with fast-axis focusing provided by lenses 34, 38, and 40) overlapping, elongated, images of the intermediate images of emitters 24 in plane 35. These overlapping images of intermediate images form a line of light 44 on a substrate 46, which among other objects or devices, can include a light modulator. It should be noted that in this system, and in similar systems discussed herein, that while fast-axis cylindrical lenses between the microlens array and the substrate do not affect beam divergence in the slow axis, the optical thickness of these lenses must be taken into effect in determining the physical location of focal planes of the spherical lens.
While the system of Reznichenko et al. serves the purpose of minimizing the above-discussed problem of emitter failure, the system has some potential shortcomings as far as uniformity and consistency of illumination along the projected line of light are concerned. This can be appreciated by considering the usual form of the distribution of light from a broad-stripe (much wider than it is high) individual; emitter and how that form is projected into the line of light.
FIG. 1A schematically illustrates an approximate hypothetical form of intensity of light in the far field of such an emitter as a function of divergence angle in a plane W just before a beam from the emitter enters microlens array 30 of the system of FIG. 1. The curve of the graph of FIG. 1 is actually a graph of a mathematical function that is a summation of two Gaussian curves displaced on negative and positive side of zero. This function is used in analyses throughout this application as the function provides for a convenient and sufficiently accurate analysis of the effects of optical systems and components on such a distribution. In this description, as is usual in the art, the term near-field is mean to designate a region within about one Rayleigh range of a beam from the emitter. The far-field is an extended region outside of the near-field region.
It should be recognized that, in practice, such a far-field distribution may not have this precise double Gaussian form, may be somewhat less symmetrical, may vary from one emitter to another, and will vary somewhat with variations in current applied to the emitter to energize the emitter. Variations in emitter-current are usually made to vary the power of light from an emitter. In this latter regard, the divergence half angle (designated angle ω in FIG. 1) can increase by up to 1° with an increase in current of about 100%. By way of example, in one prior-art line of light projector manufactured by Coherent, Inc., of Santa Clara Calif. (the assignee of the present invention) a change of emitter current from about 40 amperes (A) to about 90 A causes the slow-axis divergence half-angle (measured at the 1/e2 points of the beam) to increase from about 3.2° to about 4.5°.
In the system of FIG. 1, all of the beam from an emitter, and, accordingly, all of the hypothetical distribution of FIG. 1A enters one corresponding microlens 31 of array 30 and a corresponding microlens array 33 of array 32. The light distribution in the intermediate image is transformed from angular space to physical space by spherical lens 30 and the distribution of light as a function of distance along line of light 44 will appear (for one emitter) similar to that depicted in FIG. 1B. This is essentially the form of the far-field distribution of light from an emitter. Because of slight differences in this distribution from emitter to emitter, the distribution in the line of the summation of the contribution of all emitters will have a slightly flatter top and will exhibit modulation on a spatial frequency depending on the number of emitters contributing, among other factors. Nevertheless, in this summation, light near the ends of the line of light will be provided by light from the edges of the beams emitted by the emitters, this will result in “softness” or poor definition of the ends of the line, and also relatively strong variations in illumination at the ends of the line. This is because it is in these edges that the variation of emitter current has the most effect, and because rays in these edges are most aberrated by the microlenses.
A variation on the system of Reznichenko et al. is disclosed in U.S. Patent Application Document No. 20050063428 (Anikitchev et al.). This variation can be explained with reference again to FIG. 1, even though the slow axis spherical lens and fast-axis focusing lenses in the system of Anikitchev et al. are differently designed. In the system of Anikitchev et al. general uniformity of illumination along the line is improved by locating the front focal plane of the slow-axis spherical lens coincident with a plane (designated in FIG. 1 by dashed line 48) slightly ahead of the plane in which near field images of the emitters are formed. While this variation was effective in improving uniformity of light in general, the system suffered the same shortcoming as the system of Reznichenko et al. regarding softness and temporal variation of illumination at the ends of the line of light.
A feature in common to the systems of Reznichenko et al. and Anikitchev et al. is that microlenses in the microlens arrays are the same in number and pitch (vertex-to-vertex spacing) as emitters in the diode-laser bar, and further, are arranged such that of the beam from any emitter is accommodated by one corresponding microlens in the arrays. In an optical system disclosed in U.S. Pat. No. 6,773,142 granted to Mathew N. Rekow, and assigned to the assignee of the present invention, an attempt is made to improve uniformity in a projected line of light by configuring microlens arrays such that a beam from an emitter is wide enough in the slow axis to illuminate more than one microlens in an array, and also that there are more microlenses in an array than there are emitters, in such a way that light from more than one emitter can enter any one microlens.
A slow-axis view 50 of the system of Rekow is depicted in FIG. 2. This drawing is reproduced in part from the '142 patent, with some simplification and some added notation, and with some different reference numerals. The reference numerals are selected such that like numerals designate like features of system of FIG. 2 and the Reznichenko system of FIG. 1. It should be noted that in FIG. 2 individual emitters of diode-laser bar 22 are not designated. Further, in FIG. 2 the separate cylindrical microlens arrays are consolidated into a single element with two arrays of convex cylindrical surfaces providing the two microlens arrays. The curvature and physical aperture of the microlenses lenses depicted in FIG. 2 are greatly exaggerated.
Rekow teaches that the action of the microlens arrays is to transform the array of spaced-apart emitter apertures in diode-laser bar 22 into a virtual, single emitting aperture at or near microlens array 32. Presumably, that is near a plane indicated in FIG. 2 by dotted line 52. Rekow teaches that because the pitch of the microlenses in the microlens arrays is different from the pitch of diode-lasers in the diode-laser bar, and that because each microlens receives rays from a different plurality of emitters, rays emanating from one position in an emitting aperture will arrive in different relative positions in this virtual aperture, and that this will randomize the distribution of light in the projected line.
It has been determined that the apparatus of Rekow does provide, a generally, relatively uniform distribution of light along the projected line, but that the projected line has a softness and inconsistency at the ends that would be experienced in the above-discussed Reznichenko et al. and Anikitchev et al. systems. It has also been determined that the system is very prone to light-losses and scatter by microlens arrays. In order to determine why this is the case, a more careful analysis of how the line is formed than the simplified description provided by Rekow has been performed. Results of this analysis are discussed below with reference to FIG. 3, FIG. 4, FIGS. 5A-C, and FIG. 6.
From exemplary values of spacing, microlens-pitch (vertex-to-vertex distance between adjacent cylindrical surfaces) and curvature provided by Rekow it can be determined that a beam from any one emitter has a width at the slow-axis about sufficient to cover two microlenses. FIG. 3 schematically illustrates a fragment of the microlens arrays illuminated symmetrically by a beam 26 from an emitter. This is reproduced from an actual ray trace using ZEMAX® optical design and analysis software, available from the ZEMAX Development Corporation of Bellevue, Wash. Here, the beam completely covers a central one 31B of three microlens arrays and partially covers one microlens array on either side of the central one (31A and 31C). The incoming beam is divided into 3 parts by microlenses 31A-C. Intermediate images A, B, and C are formed by microlens pairs 31A and 33A (image A), 31B and 33B (image B), and 31C and 33C (image C). It will be immediately evident from the drawing of FIG. 3 that some rays incident on all of microlenses 31A, 31B and 31C will not reach the corresponding lenses 33A, 33B, and 33C as required for image formation, but will emerge from the microlens array via an adjacent corresponding lens and can not contribute to image formation. This light is unlikely to reach the line of light. It will also be evident that bundles of rays 54A, 54B, and 54C that converge to form images A, B, and C, respectively, are not parallel to each other and, accordingly, are not in a telecentric arrangement with lens 38.
FIG. 4 schematically illustrates the approximate form of intensity of light in the far field of an emitter as a function of divergence angle in a plane W just before a beam from the emitter enters the microlens array fragment of FIG. 3. This is assumed to have the form discussed above with respect to FIG. 1A. The angular distribution of light intensity in bundles 54A, 54B, and 54C is schematically depicted in FIG. 5A, FIG. 5B, and FIG. 5C, respectively, being approximately that of the portion of the input beam passing through the corresponding microlens pairs. Because the system is not telecentric, elongated versions of the intermediate images will not overlap in line 44 but will appear in series along the line. Accordingly, when the angular distributions of FIGS. 5A-C are transformed into physical space along the line 44, the light distribution from the emitter of FIG. 3 along line 44 will have approximately the form depicted in FIG. 6.
Now, of course, as there is a mismatch between the number and spacing of emitters and the number and pitch of microlenses in the microlens arrays, not all, if any, groups of microlenses will be symmetrically illuminated. In fact, most will be asymmetrically illuminated. Accordingly, superimposed along the line there will be edge portions A and C and center portions B from all of the emitters, but with different widths and peak intensities depending on losses. Further, some emitters that only illuminate all of two microlenses may contribute only two wide edge portions with no center portion. This seems to provide the homogenizing effect that contributes to a high degree of uniformity of illumination along the line, albeit at the expense of significant light-loss and scatter, for example, up to about 20% loss. Further, regardless of the homogenizing of the general distribution, light closest to the ends of the line 44 will come from light closest to the edges of the far field distribution of the beams from the emitters entering the microlens arrays. Accordingly, the ends of the line will have the same softness, inconsistency, and susceptibility to variation with varying emitter current, as the above-discussed systems of Reznichenko et al. and Anikitchev et al. There is a need for a line-projecting optical system that provides the beam homogenizing achieved by Rekow, but with minimized loss of light, and with minimized sensitivity to variations in intensity distribution and divergence at the edges of beams from the emitters.